To understand our present and potential future climate, we need to understand the processes that drive our climate.
The Climate System
There is a difference between the ‘climate’ and the ‘climate system’. The climate is produced by a combination of many processes in the global climate system.
The main processes of our climate system occur:
- in the atmosphere
- on the land surface (soil and vegetation)
- in the hydrosphere (ocean, rivers and lakes)
- in the cryosphere (ice and snow)
- in the biosphere.
The atmosphere is a thin layer of mixed gas that covers the Earth and prevents it becoming too hot or cold. Air takes on some characteristics of the surface beneath it. For example, in NSW, north-westerly winds tend to be warm and dry because they pass over land, whereas southerlies tend to be cool and moist because they come from the sea. The atmosphere’s circulation, the heat and light passing through it, and the processes that occur in it (such as rainfall) all affect the climate.
Different types of ground and plants at the land surface absorb varying amounts of solar energy, resulting in differing rates of evaporation and heating. The shape (topography) of the land also affects wind, slowing it down or channelling it in certain directions. In NSW, the Great Dividing Range has an influence on regional climate by affecting wind patterns, which may locally increase rain or create rain shadows, such as in the Monaro (Cooma plains). The US Earth System Research Laboratory provides more information about the influence of the land surface on climate and weather.
The hydrosphere is the oceans, rivers, lakes and ground water. The oceans are the largest component of the global climate system and influence climate by absorbing and emitting heat. Ocean currents, such as the East Australian Current, transport large amounts of heat and water around the world. Evaporation from the oceans is a major source of water vapour in the atmosphere. The hydrosphere interacts with the land surface and atmosphere by supplying ground water to plant roots, allowing transpiration.
The cryosphere is the ice that covers parts of the world—mainly sea ice in the Arctic and Southern oceans, and the land-based icesheets of Greenland and Antarctica. It also includes the ice and snow in many high-altitude regions across the globe, such as in the Snowy Mountains and on frozen land (‘permafrost’). Ice and snow are not major drivers of climate in most of NSW.
The biosphere is the collective word for parts of the Earth, including air, land, surface rocks and water, where life is found. Humans are part of the biosphere, as are cities, farms and oceans. The biosphere plays a major role in the carbon cycle and in determining the atmospheric concentration of carbon dioxide. For more about the carbon cycle, see The greenhouse effect and causes of climate change
|The global climate system includes processes on land, water, ice and air that interact to determine and change our climate. Source: Intergovernmental Panel on Climate Change 2007.|
Changes in the climate system
The climate system changes in response to both its own internal dynamics and the effects of external factors known as forcings.
The most important forcing is sunlight, which provides a constant source of energy. The Earth being a sphere tilted on its axis, the sun’s energy is concentrated on the tropics, generating heat that moves outwards through the air and water, influencing our climate system.
Like any warm object in a cold place, the Earth radiates some of the energy it receives from the sun back into space as invisible infrared radiation. About 30 per cent of this sunlight is reflected back into space; the planet absorbs the rest.
The Earth’s radiative balance or ‘energy budget’ is the difference between the amount of energy coming into the atmosphere from the sun and the amount going back out. If the balance is positive, there is warming; if negative, then cooling. If the balance is zero, the Earth is not heating or cooling.
Calculating the Earth’s radiative balance is complex, because each of the many factors affecting it has uncertainty and difficulty in measurement. For example, airborne particles (or aerosols) in the atmosphere can affect radiative balance in two ways, with bright aerosols leading to cooling and dark aerosols leading to warming. Measuring aerosols and their impacts on the Earth’s radiative balance is difficult.
Climate feedback and sensitivity
When one climatic process triggers changes to a second process, which in turn influences the first process, this interaction is called ‘climate feedback’.
A positive feedback intensifies the original process, whereas a negative feedback reduces it. Changes can happen rapidly in processes such as clouds, water vapour and sea ice, and these are called 'fast feedback'.
‘Climate sensitivity’ is a measure of how sensitive our climate is to extra greenhouse gases in the atmosphere. It is defined as how much the average global surface temperature will warm if carbon dioxide equivalents are doubled.
The climate sensitivity of the Earth is likely to be between 1.5° and 4.5°C. Every degree is important. The current global average temperature is 14°C. The difference between an ice age and a non-ice age is 6°C. In some places, regional changes will exceed the global average, whereas in others it will be below. There tends to be more warming over the land than over the sea. In Australia, annual average temperatures by 2070 are expected to rise by between 1°C (low greenhouse gas emissions scenario) and 5°C (high emissions scenario) compared with the average climate of 1980 to 1999 (CSIRO 2014).
Climate sensitivity depends mostly on positive and negative feedback effects that either strengthen or weaken the greenhouse effect. Without these feedbacks, climate sensitivity would be 1°C. The three primary feedbacks are:
- sea ice
- water vapour.
Combined with other feedbacks, these give the greatest uncertainties in predicting future climate. For example, clouds can have either a positive or negative feedback effect, depending on their altitude and the size of their water droplets. However, most scientists expect the net effect of clouds to be positive.
Sustained and truly global changes in average temperature require global heating or cooling influences through changes in:
- the heat output of the Sun
- the Earth’s orbit around the Sun
- the extent of the planet’s ice mass
- the concentration of atmospheric greenhouse gases.
Humans are able to influence only the last of these factors, but the growth in greenhouses gases in the atmosphere is significant, as discussed in the The greenhouse effect and causes of climate change.